Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The device architecture of image sensors has continued to advance at a great pace due to increasing demands for higher resolution, lower power consumption, increased dynamic range, etc. These demands have also encouraged the further miniaturization and integration of image sensors into these devices.
The typical image sensor operates as follows. Image light from an external scene is incident on the image sensor. The image sensor includes a plurality of photosensitive elements such that each photosensitive element absorbs a portion of incident image light representing a portion of the external scene to be captured. Photosensitive elements included in the image sensor, such as photodiodes, generate image charge upon absorption of the image light. The amount of image charge generated is proportional to the intensity of the image light. The image charge may be used to generate an image based on the image light.
A parameter for image sensors is dynamic range. The dynamic range of an image sensor describes the ratio between the maximum and minimum measurable image light intensities. For example, an image sensor with a low dynamic range may only be able to generate images under a very specific image light intensity, while an image sensor with a high dynamic range may be able to generate images under a variety of image light intensities. A high dynamic image sensor is desirable for most applications, especially for automotive and security applications which require image capture under a variety of image light conditions.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.